1. Field of the Invention
The present invention relates to technology for a driving circuit and a method for cold cathode fluorescent lamp (CCFL) in the backlight modules of liquid crystal displays (LCDs). In particular, this invention provides a multiple-CCFL parallel driving circuit and the associated current balancing control method for LCD.
2. Description of the Related Art
With the arriving of digital era as well as networks of information and entertainment available to millions worldwide, requesting for thin and flat panel display panels is undoubtedly an ongoing trend. In addition, features, such as larger size, better performance, and excellent uniformity of brightness, for display screens are increasingly demanded in consumer market. A LCD combined with CCFL, which works as backlight source, is so far the best choice in terms of the market requirements mentioned above. Besides, as a key component for driving light source of a flat panel display, the backlight module, directly affecting the display quality of the panel, is the basis for the reliability and stability of a CCFL's brightness.
However, as the size of LCDs becomes larger and larger, a single CCFL in the backlight module becomes less and less able to provide enough backlight. As the quantity of CCFLs in backlight module increases, many researchers are focusing on how to maintain the driving current within a reasonable tolerance range, 6 mArms±5% (or ±0.3 mArms).
R.O.C. patent No. 478292 titled “Multiple-CCFL Driving System” provides a multiple-CCFL system. The system achieves current balancing within a plurality of CCFLs by using a balance controller with the principle of impedance matching. Please refer to FIG. 1, which shows the balance control circuit of this invention using storage energy components to achieve impedance match. However, the impedance of the storage energy components cannot be precisely controlled and the temperature of the storage energy components increases after it has been operating for a long period of time, thereby affecting the performance of current balance. The effect of natural emitting heat is not good because the backlight module is enclosed in the inner space of the LCD. After a long period of operation, it cannot keep the multiple CCFLs on current balance status. Therefore, the uniformity of the image of the LCD will be reduced substantially.
R.O.C. patent No. 556860 titled “A Current Equalizer for Backlight Module” provides a current equalizer for multiple CCFLs of a backlight module. This current equalizer comprises a plurality of differential current chokes 21, 22 and 23 and capacitors 24, 25 and 26. Please refer to FIG. 2. The current of the secondary side of the differential current chokes 21, 22 and 23 reacts with the current of the primary side of the differential current chokes 21, 22 and 23 by magnetic components. By this method, this invention achieves the goal of equalizing the driving current of multiple CCFLs. Because this circuit is comprised of magnetic components and impedance components, the performance of the current balance is also affected by temperature. When this invention is applied to a large-size LCD with a larger number of multiple-CCFL, the size of the current equalizer becomes larger. This is a disadvantage when designing a light and thin LCD of a large size.
R.O.C. patent No. 485701 titled “A Cold Cathode Fluorescent Lamp (CCFL) Driving Circuit” provides a driving circuit for CCFLs. This circuit utilizes a plurality of outputs on the secondary side of the transformer to drive the multiple-CCFL and tunes the driving current of the multiple-CCFL by a feedback method. Because the impedances of the CCFLs are different from each other in the steady state, this circuit does not balance the driving current for a plurality of CCFLs.
U.S. Pat. No. 6,459,216 titled “Multiple-CCFL Current Balancing Scheme for Single Controller Topologies” provides a circuit for improving current unbalance of multiple CCFLs. This circuit samples the voltage and current of the CCFLs, gets an average value by rectifier and filter, feedbacks the sample data to a controller and adjusts the current for a plurality of CCFLs. However, the CCFLs operate under high voltage and as such this circuit needs to be concerned about anti-high voltage to prevent the component being damaged. However, it increases the costs and the size of the circuit.